It's black and arid, stupid! (9)
I believe the answer is:
brainless
'stupid' is the definition.
(synonyms)
'it's black and arid' is the wordplay.
I cannot really see how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
'black' could be 'b' (as in pencil classifications) and 'b' is found within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
The remaining letters 'rails' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for brainless that I've seen before include "Silly" , "Stupid, foolish" , "dim" , "Devoid of intelligence" .)